Used to be for free dining they would announce that officially you could do that trade, and they'd throw in some extras, like an extra snack or merchandise. This is no longer done, as the free dining is most of the year now and isn't swamping the restaurants like it used to. Some CS restaurants might still do the trade "unofficially" but some might not. You can also ask if your lobby concierge can convert your TS credits into CS credits. Where the trade is allowed, it's now a one for one trade, no extras.
On the deluxe dining plan, all credits are interchangeable for TS or CS, so the above applies only to the regular plan.
Most if not all of the trading of CS credits for multiple snacks appears to be done at resorts. It isn't for sure you'll be able to do this at a park CS, and you won't be able to do it in a store like the Main Street Confectionary as they can't accept a CS credit at all.